The size and shape of the rolling bank on a calender or mill is an important component in the calendering process. The bank provides a buffer to keep a uniform supply of material to the calender rolls. However, if the bank is not maintained at proper level, nonuniformities can appear in the calendered sheet. For example, if the bank is too low, voids can be formed in the sheet due to the "starved" condition of the bank. On the other hand, if the bank is too large, problems such as material scorching can occur, which produces cured or otherwise undesirable lumps in the sheet of material. In addition, variation in rolling bank size causes variation in the spreading force on the rolls resulting in uneven gauge of the sheet.
The ability to accurately control the size of the calender bank provides more uniform results in the calendered sheet in terms of component dimensions and composition. Inherent in the problem of controlling the bank size is the problem of being able to accurately measure the quantity and distribution of material present in the bank.
In the art, there have been a variety of methods and apparatuses employed in attempts to solve the problems associated with determining the bank size. For example, a method disclosed in U.S. Pat. No. 4,642,686 to Nagono et al. utilizes a video camera, a screen, and a light source to obtain a silhouette of the banked material. The area of the silhouette is then related to a quantity of material. The method involves a backlighting arrangement which is more complex than the inventive method.
Japanese Document No. 59-132314 to Yonezawa et al. discloses a method by which a television camera records a picture of a pair of blank rolls and then records a picture of the rolls having a bank of rubber therebetween. The difference between the pictures is obtained by a subtractor. The picture elements are then converted to black and white binary signals wherein the rubber is indicated in black. A correction means converts black portions which are below a predetermined width to white. This direct method of obtaining binary signals does not compensate for the problem of glare commonly encountered when shiny calendering rolls are employed.
European patent application 0 645 230 A1 to Hatanaka is also directed to a device for adjusting the quantity of a bank of material on a mill.
The present invention provides new and improved means for measuring and controlling the quantity of elastomeric material formed in a bank between two calendering rolls in a way which simply and effectively overcomes the problems currently encountered in the art.